Qt Creator variables

I am configuring a custom process step in Project>Build steps and want to use a working directory other than %{buildDir}.
The doc (doc.qt.io/qtcreator/creator-build-settings.html) claims there is a (Variables) button that gives a list of available variables in the current context, but none appears when I enter and edit the Working directory field. I am using Qt Creator 3.3.0 (enterprise).
Two questions: How do I get the button to appear and, failing that, what Qt Creator Variables are there other than buildDir?